This invention relates to a dynamic pressure fluid bearing device.
A herringbone-grooved bearing having grooves called herringbones which are formed in a fluid bearing portion is known as an example of a dynamic pressure fluid bearing device. An example of the shape of the fluid bearing in this type of fluid bearing device is disclosed on pages 28 to 91 and 102 to 104 of "PHILIPS RESEARCH REPORTS" No. 7, 1975. In a herringbone-grooved bearing, fluid collects inside the bearing along grooves "V" or " " shaped grooves so as to generate a dynamic pressure, thereby supporting a rotary member. A bearing which has grooves formed in its bearing portion enables a large bearing load and stable bearing performance compared with a bearing which has no grooves in its bearing portion.
However, it is expected that this type of conventional dynamic pressure fluid bearing device can be improved with respect to the points described below.
First, in the bearing device which has grooves in the form of a "V" (hereinafter referred to as "V grooves", dynamic pressure is generated by the collection of fluid which is supplied between opposed surfaces of rotary and stationary members at the center of each V along the V grooves, so that this type of bearing enables an allowable bearing load which is higher than that of a bearing having no grooves, but is considerably smaller than that of a bearing with grooves in the form of " " (hereinafter referred to as "partial grooves").
A bearing having partial grooves enables a bearing load which is larger than that of a bearing having V grooves. However, this type of bearing is not sufficiently capable of retaining the lubricating fluid. When the relative rotational speed between the shaft and the bearing portion is reduced, a contact area of the opposed surfaces of the shaft and the bearing portions increases. Therefore, the lubricating fluid does not sufficiently enter between the opposed surfaces. This raises a risk of burning the bearing surface because of a contact between the shaft and the bearing portion, or of the generation of vibration or abnormal noise. When starting or stopping, it is particularly difficult for the lubricating fluid to enter between the shaft and the bearing portion, and problems such as the above tend to occur. These problems are not negligible when a dynamic pressure fluid bearing device of the partial groove type is applied to a specific use in which it starts and stops frequently, for example, as a rotary head mechanism in a video tape recorder.